1. Field of the Invention
The present invention relates generally to transmitting and receiving video and audio signals. More particularly, the present invention relates to apparatus and method for combining audio signals into the luminance portion, and into other portions, of composite video signals to provide audio-on-video composite signals, and for separating the audio from the audio-on-video composite signals.
2. Description of the Related Art
Video is commonly transmitted by combining various synchronizing signals, or timing signals, with a luminance signal to make a composite video signal, either amplitude or frequency modulating the r.f. carrier, and broadcasting the r.f. carrier with audio on frequency modulated sidebands.
Each composite video signal includes first and second video fields of horizontal lines, with the first and second video fields interlaced. Each of the first and second video fields include a horizontal composite signal and a vertical blanking portion; and each horizontal composite signal includes a luminance portion and a horizontal blanking portion.
The horizontal blanking portion includes a front porch, a horizontal sync pulse with a leading edge, and a back porch. When color video is broadcast, a color burst is included in the back porch of each horizontal composite signal.
A plurality of horizontal composite signals is followed by a vertical blanking portion in which synchronizing and timing signals are placed to alternately produce first and second video fields of horizontal lines. The vertical blanking portions include leading equalizing pulses, a serrated vertical sync pulse, trailing equalizing pulses, and a plurality of test signals.
This prior art method of broadcasting video and audio has been very successful; but placing audio onto sidebands results in wide bandwidth requirements. For instance, if color video and audio are transmitted with the video on sidebands in a band that includes frequencies from 1850 to 1990 megahertz, and which is commonly called the 1900 megahertz band, and if both the video and audio are frequency modulated, the bandwidth requirement can exceed 25 megahertz. Or, of black and white video is transmitted with audio on sidebands, the bandwidth requirement is in the 15-18 megahertz range.
Further, while many bands have been available for entertainment video, there have been no bands available for business video. However, bands are now available that could be used for business video; but they are too narrow for conventional FM video transmission. Some of these bands are only 10 megahertz wide; and some are even narrower, being only 5 megahertz wide.
Thus, radio frequencies are available for use by businesses; and these bands could be used by a bank, or other business, for communicating by video with branches; but using prior art techniques results in a bandwidth requirement in excess of 25 megahertz for color and 15-18 megahertz for black and white video. In contrast to bandwidth requirements for video, the new bands are only 5 and 10 megahertz wide.
One solution to the requirement for a large bandwidth is to place the audio signal, or audio signals, onto the horizontal composite signal. Various schemes have been proposed to accomplish this.
Kelly, in U.S. Pat. No. 3,423,520, issued Jan. 21, 1969, places the audio signal in the horizontal blanking portion, and more particularly on the front porch. While Kelly teaches pulse amplitude modulation, he states that any form of pulse modulation may be used for the audio, including pulse duration modulation, pulse position modulation, or pulse code modulation.
Hodge, in U.S. Pat. No. 3,446,914, issued May 27, 1969, also teaches placing the audio signal in the horizontal blanking portion, but he teaches placing it on the back porch. More particularly, Hodge teaches a method for recording both audio and video by the use of a phase modulated pulse that is placed on the back porch.
Steudel, in U.S. Pat. No. 4,156,253, issued May 22, 1979, also teaches placing the sound signals onto the back porch. However, Steudel teaches placing the sound signals, in digital form, partially before the color burst and partially after the color burst.
Sugihara, in U.S. Pat. No. 4,233,627, issued Nov. 11, 1980, teaches the use of pulse code modulation to place audio signals into the vertical blanking portion. More particularly, Sugihara places the audio signals into the blanking parts that follow the equalizing pulses.
Kergosien et al., in U.S. Pat. No. 4,253,115, issued Feb. 24, 1981, teach placing audio signals in the vertical blanking portion in digital form. More particularly, Kergosien et al. teach transmission of an audio frequency analog signal during the line synchronization interval of a television signal, including coding and decoding of analog values that have ben changed to digital form.
Quan et al., in U.S. Pat. No. 4,333,108, issued Jun. 1, 1982, place the audio, in digital form, in the horizontal blanking portion. More particularly, the audio signal is pulse width modulated, thereby varying the time of at least one edge of a pulse as a function of the voltage level of the audio signal at a selected time.
Katsfey, in U.S. Pat. No. 4,361,852, issued Nov. 30, 1982, teaches placing the audio in the vertical blanking portion. The audio signal is time-compressed for injection into the vertical blanking portion. When decoded, the audio information is read into a memory device at high speed, and then read out of the memory device at low speed, thereby reconstructing a substantially continuous audio signal.
Shiral et al., in U.S. Pat. No. 4,442,461, issued Apr. 10, 1984, teach placing the audio signals on the back porch of the horizontal blanking portion in pulse-code-modulated form. In order to fit a sufficient number of digital bits onto the back porch, the back porch is extended by replacing the standard horizontal sync pulses with narrow pulses, so that the following back porch is correspondingly extended.
Hirashima, in U.S. Pat. No. 4,745,467, issued May 17, 1988, teaches placing digitized delta-encoded sound signal information in the horizontal sync part of the horizontal blanking portion, and placing a standard digital signal of the same sound signal in the vertical blanking portion.
McKenzie, in U.S. Pat. No. 4,983,967, issued Jan. 8, 1991, teaches time compressing the audio signal, and placing the time-compressed audio signal, in analog form, into the vertical blanking portion.
While all of these prior art patents achieve the objective of placing the audio onto the horizontal composite signal, none of them achieve the simplicity and the low cost that is achieved by the present invention.